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James T. Shipman Jerry D. Wilson Charles A. Higgins, Jr. Force and Motion Chapter 3

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Force and Motion – Cause and Effect In chapter 2 we studied motion but not its cause. In this chapter we will look at both force and motion – the cause and effect. This is called dynamics. We will consider Newton’s: Newton’s three laws of motion Newton’s law of universal gravitation Buoyancy and momentum Intro

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Sir Isaac Newton (1642 – 1727) Only 25 when he formulated most of his discoveries in math and physics His book Mathematical Principles of Natural Philosophy is considered to be the most important publication in the history of Physics. Intro

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Sec 3.1 Force and Net Force Force – a vector quantity capable of producing motion or a change in motion A force is capable of changing an object’s velocity and thereby producing acceleration. A given force may not actually produce a change in motion because other forces may serve to balance or cancel the effect. Section 3.1

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Sec 3.1 Force and Net Force Balanced (equal) forces produce no motion A net force produces motion Forces that are equal in magnitude but opposite in direction are said to balance each other. Section 3.1

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Sec 3.2 Newton’s First Law of Motion Aristotle considered the natural state of most matter to be at rest. Galileo concluded that objects could naturally remain in motion indefinitely. Newton’s 1 st Law of Motion – The Law of Inertia: An object will remain at rest or in uniform motion in a straight line unless acted on by an external, unbalanced force. Section 3.2

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Sec 3.2 Newton’s First Law of Motion First Law: The Law of Inertia An object will remain at rest or in uniform motion in a straight line unless acted on by an external, unbalanced force. Balanced forces have equal magnitude in opposite directions An external force is a force applied to the entire object or system. Section 3.2

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Sec 3.2 Newton’s First Law of Motion Inertia - the natural tendency of an object to remain in a state of rest or in uniform motion in a straight line (first introduced by Galileo) Basically, objects tend to maintain their state of motion and resist change. Newton went one step further and related an object’s mass to its inertia. The greater the mass of an object, the greater its inertia. The smaller the mass of an object, the less its inertia. Section 3.2

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Sec 3.2 Newton’s First Law of Motion The large man has more inertia – more force is necessary to start him swinging and also to stop him – due to his greater inertia Section 3.2

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Sec 3.3 Newton’s Second law of Motion Acceleration (change in velocity) produced by a force acting on an object is directly proportional to the magnitude of the force (the greater the force the greater the acceleration.) Acceleration of an object is inversely proportional to the mass of the object (the greater the mass of an object the smaller the acceleration.) a = F/m or F = ma Section 3.3

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Sec 3.3 Newton’s Second law of Motion b)If we double the force we double the acceleration. c)If we double the mass we half the acceleration. Section 3.3 a F m a)Original situation

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Sec 3.3 Newton’s Second law of Motion Mass = amount of matter present Earth: weight = mass x acc. due to gravity w = mg (special case of F = ma) Weight is a force due to the pull of gravity. Therefore, one’s weight changes due to changing pull of gravity – like between the Earth and Moon. Moon’s gravity is only 1/6 th that of Earth’s. Section 3.3

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Sec 3.3 Newton’s Second law of Motion Acceleration due to gravity is independent of the mass Both F and m are doubled, resulting in g remaining constant. Section 3.3

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Sec 3.3 Newton’s Second law of Motion Friction – resistance to relative motion that occurs whenever two materials are in contact with each other. Ever-present and found in all states (solids, liquids, and gases) of matter There are two types of friction: Static friction: Friction to be overcome in getting something moving from rest Kinetic friction: Friction to be overcome in keeping something moving at constant velocity In some cases we want to increase friction (sand on ice), in other cases we want to reduce friction (motor oil). Section 3.3

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Sec 3.4 Newton’s Third Law of Motion For every action there is an equal and opposite reaction. or Whenever on object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. action = opposite reaction F 1 = -F 2 or m 1 a 1 = -m 2 a 2 Section 3.4

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Sec 3.5 Newton’s Law of Gravitation Gravity is a fundamental force of nature We do not know what causes it We can only describe it Law of Universal Gravitation – Every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them Section 3.5

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Sec 3.5 Newton’s Law of Gravitation G is the universal gravitational constant G = 6.67 x 10 -11 N. m 2 /kg 2 G: is a very small quantity thought to be valid throughout the universe was measured by Cavendish 70 years after Newton’s death not equal to “g” and not a force Section 3.5 Gm 1 m 2 r 2 Equation form: F =

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Sec 3.5 Newton’s Law of Gravitation The forces that attract particles together are equal and opposite F 1 = -F 2 or m 1 a 1 = -m 2 a 2 Section 3.5

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Sec 3.5 Newton’s Law of Gravitation For a homogeneous sphere the gravitational force acts as if all the mass of the sphere were at its center Section 3.5 Gm 1 m 2 r 2 F =

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“ Sec 3.5 Newton’s Law of Gravitation Weightlessness in space is the result of both astronaut and the spacecraft ‘falling’ to Earth at the same rate Section 3.5

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Sec 3.6 Archimedes’ Principle and Buoyancy Buoyant force – the upward force resulting from an object being wholly or partially immersed in a fluid. The Archimedes’ Principle – An object immersed in a fluid experiences a buoyant force equal to the weight of the volume of fluid displaced. Both liquids and gases are considered fluids. Section 3.6

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Sec 3.6 Archimedes’ Principle and Buoyancy The buoyant force depends on the weight of the fluid displaced. The weight of the fluid displaced depends on the density of the fluid and the volume displaced. Salt water is denser than fresh water. Therefore, one floats higher in salt water, since one needs to displace less salt water to equal one’s weight. Section 3.6

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Sec 3.7 Momentum Linear momentum = mass x velocity p = mv If we have a system of masses, the linear momentum is the sum of all individual momentum vectors. P f = P i (final = initial) P = p 1 + p 2 + p 3 + … (sum of the individual momentum vectors) Section 3.7

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Sec 3.7 Momentum Law of Conservation of Linear Momentum - the total linear momentum of an isolated system remains the same if there is no external, unbalanced force acting on the system Linear Momentum is ’conserved’ as long as there are no external unbalanced forces. It does not change with time. Section 3.7

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Sec 3.7 Momentum P i = P f = 0 (for man and boat) When the man jumps out of the boat he has momentum in one direction and, therefore, so does the boat. Their momentums must cancel out! (= 0) Section 3.7

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Sec 3.7 Momentum Angular Momentum, L = mvr L = angular momentum, m = mass, v = velocity, and r = distance to center of motion Torque – the twisting effect caused by one or more forces As we have learned, the linear momentum of a system can be changed by the introduction of an external unbalanced force. Similarly, angular momentum can be changed by an external unbalanced torque. Section 3.7

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Sec 3.7 Momentum Torque varies with the length of the lever arm. As the length of the lever arm is doubled, the torque is doubled, for a given force. Section 3.7

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Sec 3.7 Momentum Law of Conservation of Angular Momentum – The angular momentum of an object remains constant if there is no external, unbalanced torque (a force about an axis) acting on it Concerns objects that go in paths around a fixed point, for example a comet orbiting the Sun Section 3.7

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Sec 3.7 Momentum Mass (m) is constant. Planet orbit paths are slightly elliptical, therefore both r and v will slightly vary during a complete orbit. As r changes so must v. When r decreases, v must increase so that m 1 v 1 r 1 = m 2 v 2 r 2 Section 3.7

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